A scanning optical device used for a LBP (Laser Beam Printer) and others scans a bundle of rays from a semiconductor laser element by using a light deflector, and has an effect of imaging on a photoconductor drum by using a scanning optical system, wherein the semiconductor laser element is a light source and the photoconductor drum is a surface subjected to scanning. In such scanning optical device, when there is a change in an environmental temperature, a wavelength of a bundle of rays and a refractive index of a lens element included in an optical system change accordingly. Thus, the scanning optical device is required to be designed such that a movement of a focal position in accordance with an environmental temperature change is sufficiently reduced. For example, in the scanning optical device there is used a lens system for parallelizing a divergent bundle of rays radiated from a semiconductor laser element, or converting into a convergent bundle of rays (the lens for parallelization is specifically called “collimate lens”), and for such lens system there is generally used a glass lens element having a small refractive index change against a temperature change.
On the other hand, in Japanese Laid-Open Patent Publication No. 04-328512 there is proposed a method of further simplifying this lens system by combining a lens having a diffraction effect with a laser light source, and reducing a deviation in an imaging position due to a temperature change by fixing a focal length.
In addition, in Japanese Laid-Open Patent Publication No. 04-328516 is disclosed a method of using a Fresnel lens for the collimate lens, and compensating defocus of the scanning optical system due to a temperature change by combining: a wavelength change of a laser light source in accordance with a temperature change; a focal length change of the Fresnel lens; and a change due to an expansion in a lens barrel member of the Fresnel lens.
Furthermore, in Japanese Laid-Open Patent Publication No. 06-118346 there is disclosed a method of offsetting a change in a focal length of the collimate lens against temperature by combining a Fresnel lens with a refractive lens.